Summary:
Mineral deposits in the Las Chispas District are classified as low to intermediate sulfidation gold and silver epithermal systems, typical of many deposits in northeastern Sonora.
The host rocks to mineralization in the Las Chispas district are generally pyroclastic, tuffs, and rhyolitic flows interpreted to be members of the Lower Volcanic Complex. Locally, volcanic pyroclastic units mapped within the underground workings include rhyolite, welded rhyodacite tuff, lapilli (lithic) tuff, and volcanic agglomerate.
Generally, the host rocks are above the existing water table. Oxidation of sulphides is observed from near-surface to depths greater than 300 m and the presence of secondary minerals is recorded from the Las Chispas historic underground workings approximately 60 m to 275 m in depth from the surface. Strong and pervasive near-surface oxidation is noted to occur in the Babicanora Area, where host rocks experienced faulting and advanced weathering to limonite, hematite, and clays.
Regionally, the Las Chispas Operation is situated in an extension basin related to a Late Oligocene half-graben of the Sonora River Basin. Multiple stages of normal faulting affect the basin. The main structures are steep, west-dipping (80°) and sub-parallel to the Santa Elena-Las Chispas normal fault, which is located along the western margin of the Las Chispas Operation, striking approximately 210°. The basin is further cross-cut by younger northwest–southeast trending normal faults that dip to the southwest, creating both regional and local graben structures. Locally, the graben structures are complicated by the effects of probable caldera collapse. Three structural controls, excluding bedding contacts, are considered to influence alteration and mineralization:
• 150–170° striking and are inclined at approximately 65–75° to the southwest;
• 340–360° striking and are inclined 75° west to 75° east; and
• 210–230° striking and are inclined 70–85° to the northwest.
Mineralization is interpreted to be a deeply emplaced, low- to intermediate-sulfidation system, with mineralization hosted in hydrothermal veins, stockwork, and breccia. Emplacement of the mineralization is influenced by fractures and low-pressure conduits formed within the rocks during tectonic movements. Mineralization can be controlled lithologically along regional structures, local tension cracks, and faulted bedding planes.
Historical reports and more recent work have investigated the gold, silver, base metals, and gangue minerals associated with the mineralization. The mineralization is 0.10 m to 9.30 m in true thickness and typically encompasses a central quartz ± calcite mineralization corridor with narrow veinlets within the adjacent fault damage zone. Stockwork and breccia zones are centered on structurally controlled hydrothermal conduits.
Historical reporting has identified economic mineralization in the form of silver sulfides and sulfosalts as the primary silver mineral species, and in association with pyrite. Secondary silver enrichment is indicated by the gradation from chlorargyrite near the surface to pyrargyrite at depth.
Silver mineralization dominates throughout the Las Chispas Operation. Ratios of silver to gold using a cut-off grade (“COG”) of 150 g/t silver equivalent (“AgEq”) are approximately: 90:1 at Babicanora Main Vein, 89:1 at Babi Vista Main Vein, 117:1 at Babicanora Norte Main Vein, 56:1 at Babicanora Sur Main Vein, 102:1 at Granaditas Vein, 142:1 at Las Chispas Vein, 172:1 at Giovanni Vein, and 140:1 at William Tell Vein. Overall, a 1:100 gold to silver modal ratio is considered average for the Las Chispas Operation.
Gold mineralization is stronger within the Babicanora Area than within the Las Chispas Area. The modes of gold mineralization currently identified are threefold:
1) gold associated with pyrite and chalcopyrite;
2) gold emplacement with silver sulfides (typically argentite and electrum); and
3) native gold flakes in quartz.
Additional sulfide minerals present are minor chalcopyrite, sphalerite and galena. The veins are low in base metal mineralization, except for the far south-eastern extensions of the Babicanora Norte, Babi Vista and Granaditas veins, in the southeastern part of the District. In addition to the petrographic findings in Babicanora, samples of an early sphalerite phase were followed by a later galena phase of mineralization and visual inspection of the base metal mineralization showed galena and sphalerite emplaced at the same time within the same discrete vein. Multiple pulses of base metal-rich fluids of variable composition formed the mineralization at the Las Chispas Operation. There appears to be an increasing base metal content to the southeast and at depth. Government geophysical maps show a large magnetic anomaly to the east of the Las Chispas Operation area, which could be a buried intrusion and potentially the main source of the mineralization in the District.
The veins and stockwork within the Las Chispas Vein consist of fine- to medium-grained, subhedral to euhedral interlocking quartz with minor cavities lined by comb quartz (typically crystals are 5 to 10 mm in length). Quartzpseudomorphed blades after platy carbonate or other textures that indicate a shallow environment have not been observed. Vein emplacement and form are structurally and lithologically controlled. The rheology of the host rock plays an important role in structural preparation and emplacement of the mineralization. Within the fine-grained welded tuff, veining is narrow, typically with sharp narrow contacts. Veins and breccia emplaced in the more competent, medium-grained lapilli tuffs are wider and commonly occur with parallel splays along the main structure and denser veining in the adjacent fault damaged rocks.
Brecciated mineralization formed in two ways: 1) in zones of low pressure as hydrothermal breccia: and 2) as mechanical breccias.
Mineralization in the hydrothermal breccia is hosted in a siliceous matrix of hydrothermal quartz ± calcite and previously formed vein clasts that have been brecciated and re-cemented. Clasts are typically monolithic, angular, and show minimal signs of milling and rounding by hydrothermal processes. Although heterolithic breccias are present, they tend to be at the intersection points of the cross-cutting faults (striking 360°) to the main trend and at depth. Where breccia clasts are mineralized, mobilization of the clasts within conduits during multi-episodic pulse events is indicated. Gold values increase with increasing amounts of pyrite and chalcopyrite within the quartz matrix.
Re-cemented mechanical breccia, generated by the reactivation of the fault hosting the mineralization, is also present. These breccias consist of fault gouge, have a cataclasite texture, and are re-cemented with quartz and calcite. This reactivation mechanism also produces open space filling ores, including narrow stockwork quartz ± calcite ± adularia veins. Additional textures present are banding, crustiform, comb, and chalcedonic silica-calcite veins. The matrix commonly has fine disseminated to coarse-grained banded sulfides associated with the cement.
Argentite is the principal silver mineral and occurs in association with galena, pyrite ± marcasite, and chalcopyrite. Gold and silver values have a strong positive correlation with each other and likely precipitated together during crystallization of the quartz. Base metals contents are low in veins. Minor zinc and lead are principally found as blebs and veinlets in black sphalerite and galena. Antimony is minor and arsenic and mercury are conspicuously absent. Minor secondary copper minerals, such as chrysocolla and malachite, occur underground in association with oxidized chalcopyrite, but are rare.